Photoconductive composition employing an interpolymer blend as binder material

ABSTRACT

DISCLOSED HEREIN ARE PLOYMER BLENDS COMPRISING A FIRST INTERPOLYMER OF A VINYL ALCOHOL ESTER AND AT LEAST ONE ETHYLENICALLY UNSATURATED ACID ESTER AND A SECOND INTERPOLYMER COMPRISING A VINYLIDENE AROMATIC COMPOUND, AT LEAST ONE ETHYLENICALLY UNSATURATED ACID ESTER AND AT LEAST ONE COMPOUND CONTAINING ONE OR MORE ACID GROUPS. THESE BLENDS ARE PARTICULARLY USEFUL AS BINDERS AND ADHESIVES IN PHOTOCONDUCTIVE COMPOSITIONS.

United States Patent i 3,745,906 PHOTUCUNDUC'HVE COMPOSKTION EMPLOY- ING AN ENTERPOLYMER BLEND AS BINDER MATEREAL Robert B. Elance, East Longmeadow, David R. Cahill,

Wilbrahain, and Peter Wachtei, Springfield, Mass, assignors to Monsanto Company, St. Louis, Mo.

No Drawing. Original application Dec. 13, 1968, Ser. No. 733,708, new Patent No. 3,607,376. Divided and this application Apr. 21, 1971, der. No. 136,2fi9

Int. Cl. G03g 5/08 U5. El. 96-18 8 Ilaims ABSTRACT OF THE DECLOSURE Disclosed herein are polymer blends comprising a first interpolymer of a vinyl alcohol ester and at least one ethylenically unsaturated acid ester and a second interpolymer comprising a vinylidene aromatic compound, at least one ethylenically unsaturated acid ester and at least one compound containing one or more acid groups. These blends are particularly useful as binders and adhesives in photoconductive compositions.

This application is a division of application Ser. No. 783,700, filed Dec. 13, 1968, and now U.S. Patent 3,607,- 376, issued Sept. 21, 1971.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to novel blends of interpolymers which are useful as binders and adhesives. More particularly, this invention relates to blends of a first interpolymer comprising a vinyl alcohol ester and at least one ethylenically unsaturated acid ester and a second interpolymer comprising a vinylidene aromatic compound, at least one ethylenically unsaturated acid ester and at least one ethylenically unsaturated compound containing at least one acid group.

(2) Description of the prior art Electrophotography relates to substrates having a photoconductive substance coated thereon which provides a surface upon which images; are preproducible. The substrate may, for example, be paper, glass, metal foil, etc. The photoconductive substance is a material having electrical conductivity dependent upon the amount of light to which it is subjected. The photoconductive layer, although not light sensitive per se, is rendered so by applying to the coated article an electrostatic charge (e.g., by Corona discharge). Images to be reproduced are placed on top of the charged surface which is exposed to illumination by conventional photographic methods, whereby the coating locally dissipates its charge in proportion to the intensity of illuminaton. Non-illuminated areas, which retain their charge, are then treated with electroscopic powder (e.g., colored synthetic reins) resulting in a visible image which may be transferred to another surface or may be fixed, e.g., by fusing, rendering the powder permanently adherent to the coating substrate.

Fundamentally, then, electrophotographic coatings comprise a photoconductor such as, for example, zinc oxide, which is bonded by an organic resin to a base sheet or other substrate. It is known that copolymers of vinyl acetate and copolymers of styrene are individually effective as binders. Coatings based upon vinyl acetate copolymers, however, have been found to be deficient in speed. Further, coatings of copolymers of styrene have been found to be low in charge acceptance and yield copies of poor image density. Both deficiencies of the separate copolymers may be overcome, at least in part,

dfldidhd Patented July ltd, 1973 by blending the two types of copolymers. The major drawback of such blends, however, has been their incompatibility as exhibited by their rapid phase separation. Such separation is a serious drawback to commercial utilization. It might be expected that compatible blends useful in this area could be achieved by the mere matching of solubility parameters. Attempts to achieve this, however, have been unsuccessful. It is necessary for commercial utility that the blended resins be compatible in solution concentrations of at least 20% by weight. Imposed upon this requirement for compatibility, however, is, of course, the requirement that the blend employed adhere to the particular photoconductive element being used in the system and to the substrate as well. In addition, the binder employed must enhance the photoconductive properties of the system.

SUMMARY OF THE INVENTION This invention relates to a composition of matter comprising a first interpolymer of at least one vinyl alcohol ester and at least one ethylenically unsaturated acid ester and a second interpolymer comprising at least one vinylidene aromatic compound, at least one ethylenically unsaturated acid and at least one ethylenically unsaturated compound containing at least one acid group.

More particularly, this invention relates to a composition of matter comprising a blend of:

(A) a first interpolymer comprising at least one vinyl alcohol ester and at least one ethylenically unsaturater acid ester containing at least 7 carbon atoms and (B) a second interpolymer comprising (1) at least one vinylidene aromatic compound,

(2) at least one ethylenically unsaturated acid ester,

and

(3) 0.5% to 5.0% by weight, based upon the total weight of the second interpolymer, of at least one ethylenically unsaturated compound containing at least one acid group.

This invention also relates to the use of such blends in various adhesive compositions wherein the term adhesive is construed as including binders, and in various photoconductive compositions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The vinyl alcohol ester of the first interpolymer of the blends of this invention may be any vinyl alcohol ester which is copolymerizable with the ethylenically unsaturated acid ester containing at least 7 carbon atoms. The choice of the particular vinyl alcohol to be employed will generally depend upon the pigment which is to be included and the substrate to be coated. That is to say, the interpolymer produced should have the ability both to wet the pigment and to adhere to the substrate. Examples of such vinyl alcohol esters are vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pentanoate, vinyl hexanoate, vinyl heptanoate, vinyl octanoate, etc., their isomers and mixtures thereof. Where the compositions of this invention are to be used as binders in electrophotographic processes, it will usually be desirable that they be able to wet zinc oxide, a commonly used photoconductive pigment. Where this is the case it will generally be advantageous that a lower vinyl alcohol ester, e.g. vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, or isomers thereof be employed. Thus, these four vinyl alcohol esters are the preferred vinyl alcohol esters in the practice of this invention. Vinyl acetate is most preferred because of its ready commercial availability and favorable capacity for interpolymertization.

A second component of the first interpolymer of the blends of this invention is an ethylenically unsaturated acid ester containing at least 7 carbon atoms. It is this component that is critical to the attainment of compatibility between the two interpolymers. Such esters may of course be prepared by the reaction of organic acids and alcohols. The acid moiety of such esters may be monocarboxylic, polycarboxylic, monosulfonic, or polysulfonic acids such as, for example, acrylic acid, methacrylic acid, maleic acid, fumaric acid, crotonic acid, vinyl sulfonic and the like. The alcohol moiety of the ester may be any alcohol containing suflicient carbon atoms to bring the total number of carbon atoms of the ester moiety to at least 7. Thus the alcoholic moiety may, for example, be methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, etc., their isomers and mixtures thereof. Of those compounds which may be used as the second component of the first interpolymer, acrylic and methacrylic acid esters are preferred. In particular, it has been found that butyl acrylate provides optimal properties for electrophotographic coatings when interpolymerized with vinyl acetate and is thus the most preferred ester.

In addition to the two critical components of the first interpolymer described above, other monomers may be interpolymerized therewith to impart particular desirable qualities to the resin. In particular, it has been found beneficial and desirable in photoconductive electrophotographic applications that an ethylenically unsaturated acid ester containing 4-6 carbon atoms be included, for example, methyl acrylate, ethyl acrylate, propyl acrylate, methyl methacrylate, ethyl methacrylate, and the like. Other esters such as dimethyl maleate, dimethyl tumarate and the like may be employed.

The vinyl aromatic compound moiety of the second interpolymer is preferably styrene. However, ring-substituted styrenes in which the substituents are 1-4 carbon atoms alkyl groups or chlorine atoms may be employed. Examples of such ring-substituted styrenes include ortho-, metaand para-methyl, ethyl, butyl, etc., mono-al'kyl styrenes, 2,3-2,4-dimethyl and diethyl styrenes; mono-, diand trichlorostyrenes; alkyl chlorostyrenes such as 2- methyl-4-chlorostyrene, etc. The use of two or more such styrene monomer moieties is also within the scope of the invention. The ethylenically unsaturated acid ester which constitutes a second component of the second interpolymer may be any alkyl ester of an ethylenically unsaturated monocarboxylic or polycarboxylic acid, e.g., methyl acrylate, methyl methacrylate, methyl crotonate, ethyl acrylate, ethyl methacrylate, ethyl crotonate, dimethyl maleate, dimethyl fumarate, diethyl maleate, dipropyl itaconate, butyl crotonate, and the like. The preferred ester is ethyl acrylate.

The second interpolymer further contains at least one ethylenically unsaturated compound containing at least one acid group. This acid group may be a carboxylic acid group of a sulfonic acid group. Examples of compounds which may be employed are acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid; mono esters and partial mono esters of maleic acid, fumaric acid, itaconic acid, e.g., mono methyl maleate, monoethyl fumarate, etc.; vinyl sulfonic acid, styrene sulfonic acid, sulfoethyl methacrylate, sulfoethyl acrylate, sulfoethyl maleate, and the like.

Any of various pigments may be dispersed in the compositions of this invention so long as it has adhesive affinity for the particular composition employed. By adhesive alfinity is meant that the composition will adhere to the pigment and act as a binder therefor. Exemplary of the many pigments which might be used are titanium dioxide, ferric oxide, graphite, lead oxide, lead chromate and the like. When the compositions of this invention are to be used as binders in electrophotographic processes the pigment will be one capable of being rendered light sensitive by the application thereto of an electrostatic charge. As examples of such photoconductors may be listed antimouy oxide, aluminum oxide, bismuth oxide, cadmium oxide, mercury oxide, molybdenum oxide, lead oxide, zinc iodide, cadmium iodide, cadmium sulfide, selenium metal, arsenic trisulfide, lead telluride and the like. In particular, zinc oxide is preferred for such application. Mixtures of two or more pigments may also be employed.

For such electrophotographic processes, various organic photoconductors may also be used either in addition to the inorganic, e. g. zinc oxide, or in place of them. Aromati'c amines may, for example, be employed for this purpose. Among the many that may be used are: 4,5- diphenyl-Z-imidazolone, 4,5 diphenylimidazole-Z-thione and their derivatives, 1-phenyl-3,5-bis(p-methoxystyryl) pyrazoline, polymers of N-vinyl carbazole, 4,4"-dibromoterphenyl, poly(9-vinyl anthracene), etc.

The following examples are presented in illustration of the invention but are not to be construed as limitations thereof. Where parts are mentioned, parts by weight are intended unless otherwise indicated.

EXAMPLE 1 Part A Into a reaction vessel equipped with an agitator, a condenser, a thermometer and a metering pump for controlled, delayed monomer input is introduced 500' parts of vinyl acetate, 14 parts of ethyl acrylate, 14 parts of butyl acrylate, parts toluene, 5 parts of benzoyl peroxide and 2 parts of t-butyl perbenzoate. The mixture is then heated to reflux. In a separate container is mixed 236 parts of ethyl acrylate, 236 parts of butyl acrylate and 200 parts of toluene. This mixture is then introduced into the reservoir supplying the metering pump and addition of the delay and monomer charge is begun. The delay is carried out over 4-40 minute periods. During the first 40 minute period 306 parts of the delayed monomer mixture is added, during the second 40 minute period 210 parts, during the third 40 minute period 107 parts and, finally, during the last 40 minute period the final 50 parts of delay is added. The batch is then held at reflux for an additional 4 hours. The solution is diluted with 230 parts of toluene and then cooled to room temperature. The resulting polymer containing solution has a total solids content of 67% and a Brookfield viscosity (N0. 4 spindle at 12 r.p.m.) of 35,000.

Part B Into a reaction vessel equipped as in Part A above is introduced parts of styrene, 250 parts of ethyl acrylate, 200 parts of toluene, 1.0 part of acrylic acid and 4 parts of t-butyl perbenzoate. The batch is heated to reflux. A delay charge consisting of 75 parts of styrene, parts of toluene and 4.0 parts acrylic acid is mixed in a separate vessel. The delay is added over a 4 hour period. During the first hour 136 parts of delay is added, during the second hour 67 parts, during the third hour 34 parts and finally, during the fourth hour 17 parts. The batch is held at reflux for an additional 16 hours and then cooled to room temperature yielding a polymer solution having a total solids of 58.2% and a Brookfield viscosity (No. 4 spindle at 40 r.p.m.) of 7,100.

Part C 20 parts of the 67 solids solution of Part A is mixed with 23 parts of the 58% solids solution of Part B and 5.7 parts of toluene is added thereto to yield a 55% solids solution of the blends. Visual examination shows that this solution blend is compatible.

PartD 68 parts of the 55% solution prepared in Part C is weighed into a Waring Blendor. 144 parts of toluene is added and the new mixture is well agitated. Next is added 225 parts of a French process zinc oxide and agitation is continued at high speed for 7-10 minutes. 4.5 parts of a /z% solution in methanol of a sensitizing dye is then added with thorough mixing. The dispersion thus prepared is applied to a commercially available conductive base-stock paper to a coating weight of 20 lbs. per ream. The coated paper is then dried and conditioned at 50% relative humidity in the dark and then tested for charge acceptance, dark decay charge and decay of charge during illumination using a Victoreen Model EPA-825 electrostatic paper analyzer. The procedure by which this is accomplished is set forth in RCA Review, December, 1961, pages 780-790. For the coating prepared in this example a charge acceptance is 440 volts, the dark decay of charge is 7.5 volts per second and the decay of charge during illumination is 104 volts per second.

EXAMPLE 2 Control Example 1 is repeated using the interpolymer of Example 1, Part A and substituting for the interpolymer of Example 1, Part B an interpolymer containing no acrylic acid, i.e., 50/50 styrene/ ethyl acrylate rather than 49/ 5 /1 styrene/ ethyl acrylate/ acrylic acid. The blended resin solutions are incompatible and no coating is obtainable upon which to test the electric properties.

EXAMPLE 3 Example 1 is repeated substituting for the interpolymer of Part B an interpolymer of 49.5/50/0.5 styrene/ethyl acrylate/ acrylic acid. Again, the blend is found to be compatible to toluene and the electrical properties are: charge acceptance, 425 volts; dark decay of charge, 10 volts per second; and decay of charge during illumination, 94 volts per second.

EXAMPLE 4 Example 1 is repeated substituting for the interpolymer of Part B an interpolymer of 45/50/50 styrene/ethyl acrylate/acrylic acid. This blend is compatible in a solution with toluene and n-propanol and the electrical properties are 515 volts charge acceptance, 5.0 volts per second dark decay and 135 volts per second illuminated decay.

EXAMPLE 5 Example 1 is repeated except that a 60/40 vinyl acetate/ butyl acrylate copolymer is substituted for the 50/25/25 vinyl acetate/ ethyl acrylate/butyl acrylate interpolymer employed therein. The blend is found to be compatible in toluene solution and the electrical properties are 441 volts charge acceptance, 7.0 volts per second 'dark decay and 86 volts per second illuminated decay.

EXAMPLE 6 Control Example 1 is repeated except that a copolymer of 70/30 vinyl acetate/methyl acrylate is substituted for the interpolymer of Part A. This time the blend is incompatible in toluene solution and the charge acceptance is only 166 volts. Dark decay is 5.0 volts per second and illuminated dacay is 94 volts per second. This example demonstrates the deficiencies of a system not containing an ethylenically unsaturated acid ester containing at least 7 carbon atoms as a part of the first interpolymer.

EXAMPLE 7 Into a reaction vessel is charged 180 parts styrene, 250 parts of ethyl acrylate and 1 part of 2-sulfoethyl methacrylate, 370 parts of toluene and 8 parts of t-butyl perbenzoate. An additional 65 parts of styrene and 4 parts of sulfoethyl methacrylate blended With 5 parts of toluene are added to the reactigp in two separate streams over a 4 hour period while the batch is held at reflux. The reaction is maintained at reflux for an additional 20 hours and is cooled yielding a polymeric solution of 67.5% solids. The Brookfield viscosity is 1,600 centipoises (No. 3 spindle at 30 r.p.m.). The resulting interpolymer is then substituted for the interpolymer of Example 1, Part B and the final product is found to have a charge acceptance of 448 volts, a dark decay of 4.0 volts per second and an illuminated decay of 107 volts per second.

EXAMPLE 8 Example 1 is repeated except that an interpolymer of 47/50/3 chlorostyrene/ethyl acrylate/ acrylic acid is substituted for the interpolymer of Part B. The blend is again found to be compatible in toluene and the electrical properties are 177 volts charge acceptance, 4.5 volts per second dark decay and 87 volts per second illuminated decay.

It will be noted that in the foregoing examples the first and second interpolymers have been blended in 1:1 ratios. It is not necessary to the practice of this invention that such be the case. The 1:1 ratio was chosen as being the most rigorous test of compatibility; that is to say, it compatibility can be shown at 1:1, it will also be achievable where one or the other of the components is present in major proportions. For particular applications as, for example, in electrophotography optimization of a particular system well may dictate variations in proportion as divergent as 9:1 or 1:9 or even beyond.

Further, it should be noted that although the examples show only application to paper substrates, they may also be successfully applied to metal, glass, ceramics and, in particular, wooden substrates. Thus, upon formulation with the various pigments, fillers, driers, extenders, dyes, etc., well known to those of skill in the art, paints and other coatings for application to a wide variety of surfaces may be produced.

Thus, it will be obvious that many variations and modifications may be applied to the invention described herein without departing from the scope thereof.

What is claimed is:

1. In a method for reproducing images upon a surface which comprises coating an electrically conductive substrate, applying an electrostatic charge whereby the coating is rendered light sensitive, illuminating the charged surface and treating with electroscopic powder whereby a visible image results, the improvement which comprises employing as the surface coating a dispersion of at least one photoconductive pigment in an interpolymer blend comprising:

(A) a first interpolymer consisting of an interpolymerization product of at least one vinyl alcohol ester and at least one ester of acrylic acid containing at least seven carbon atoms and (B) a second interpolymer consisting of an interpolymerization product of:

(1) at least one vinyl aromatic compound,

(2) at least one ester of acrylic acid and (3) 0.5 to 5.0% by weight, based upon the total weight of the second interpolymer, of at least one ethylenically unsaturated compound containing at least one acid group wherein the weight ratio of the first interpolymer to the second interpolymer is in the range from 9:1 to 1:9.

2. The improved method of claim ll wherein the first interpolymer comprises an interpolymerization product of:

(A) at least one vinyl alcohol ester,

(B) at least one ethylenically unsaturated acid ester containing at least seven carbon atoms and (C) at least one ethylenically unsaturated acid ester containing 4-6 carbon atoms.

3. The method of claim 1 wherein the surface is paper.

4. The method of claim 1 wherein the photoconductive pigment is zinc oxide.

5. A composition comprising a blend of:

(A) a first interpolymer consisting of an interpolymerization product of at least one vinyl alcohol ester and at least one ester of acrylic acid containing at least seven carbon atoms,

7 8 (B) a second interpolymer consisting of an interpolymtotal weight of the second interpolymer of at erization product of: least one ethylenically unsaturated compound (1) at least one vinyl aromatic compound, containing at least one acid group, and (3) at least one ester of acrylic acid and (C) at least one photoconductive pigment having adhe- (3) 0.5 to 5.0 percent by Weight, based upon the 5 sive affinity for the interpolymer blend, dispersed total weight of the second interpolymer of at within the blend; least one ethylenically unsaturated compound wherein the weight ratio of the first interpolymer to the containing at least one acid group, and second interpolymer is in the range from 9:1 to 1:9. (C) at least one photoconductive pigment having adhe- 8. The article of claim 7 wherein the photoconductor sive affinity for the interpolymer blend, dispersed m is zinc oxide. Within the blend; References Cited wherein the weight r at1 0 of the first lnterpolymer to the UNITED STATES PATENTS second interpolymer 1S 1n the range from 9:1 to 1:9.

6. The composition of claim 5 wherein the photocon- 2,934,510 4/1960 Cnssey et a1 117-75 X ductor is zinc oxide 15 3,037,955 6/1962 Carman 1l716l X 7. An article of manufacture comprising an electrically $128,204 4/1964 schaum et a1 X conductive substrate having deposited thereon a blend 3,378,370 4/ 1968 Brancato comprising; 3,481,735 12/1969 Graver et a1. 96-15 (A) a first interpolymer consisting of an interpolymeri- 493,369 2/1970 Busch et zation product of at least one vinyl alcohol ester and 20 3,540,886 11/1970 Ansel et 961-8 at least one ester of acrylic acid containing at least 3,607,376 9/1971 Blame et a1 117-155 UA seven carbon atoms, (B) a second interpolymer consisting of an interpolym- RONALD SMITH Primary Exammer erization product of: J. R. MILLER, Assistant Examiner (1) at least one vinyl aromatic compound, 25 (2) at least one ester of acrylic acid and (3) 0.5 to 5.0 percent by weight, based upon the 96-1.5; 252--501 US. Cl. X.R. 

